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Carbon transitions from either Calvin cycle or transitory starch to heteroglycans as revealed by ¹⁴C‐labeling experiments using protoplasts from Arabidopsis

Malinova, Irina, Steup, Martin, Fettke, Joerg
Physiologia plantarum 2013 v.149 no.1 pp. 25-44
Arabidopsis thaliana, Calvin cycle, carbon, lighting, monosaccharides, mutants, phosphoglucomutase, protoplasts, starch, transferases
Plants metabolize transitory starch by precisely coordinated plastidial and cytosolic processes. The latter appear to include the action of water‐soluble heteroglycans (SHGᵢₙ) whose monosaccharide pattern is similar to that of apoplastic glycans (SHGₑₓ) but, unlike SHGₑₓ, SHGᵢₙ strongly interacts with glucosyl transferases. In this study, we analyzed starch metabolism using mesophyll protoplasts from wild‐type plants and two knock‐out mutants [deficient in the cytosolic transglucosidase, disproportionating isoenzyme 2 (DPE2) or the plastidial phosphoglucomutase (PGM1)] from Arabidopsis thaliana. Protoplasts prelabeled by photosynthetic ¹⁴CO₂ fixation were transferred to an unlabeled medium and were darkened or illuminated. Carbon transitions from the Calvin cycle or from starch to both SHGᵢₙ and SHGₑₓ were analyzed. In illuminated protoplasts, starch turn‐over was undetectable but darkened protoplasts continuously degraded starch. During illumination, neither the total ¹⁴C content nor the labeling patterns of the sugar residues of SHGᵢₙ were significantly altered but both the total amount and the labeling of the constituents of SHGₑₓ increased with time. In darkened protoplasts, the ¹⁴C‐content of most of the sugar residues of SHGᵢₙ transiently and strongly increased and then declined. This effect was not observed in any SHGₑₓ constituent. In darkened DPE2‐deficient protoplasts, none of the SHGᵢₙ constituents exhibited an essential transient increase in labeling. In contrast, some residues of SHGᵢₙ from the PGM1 mutant exhibited a transient increase in label but this effect significantly differed from that of the wild type. Two conclusions are reached: first, SHGᵢₙ and SHGₑₓ exert different metabolic functions and second, SHGᵢₙ is directly involved in starch degradation.